The tithe applotment records for Clonduff in 1834 list just one Truesdale, Francis in Ballynagappoge. However there may have been others living there of that surname without land. They wouldn’t be listed.
http://www.irishgenealogyhub.com/down/tithe-applotment-books/clonduff-parish.php#.WIAEsBR2ug0
Griffiths Valuation for 1864 lists 2 Truesdale farms in Ballynagapogue. Thomas had plot 2a a 27 acre farm. Today that’s on the Bannfield Rd. Samuel had plot 30 which was an 11 acre farm on today’s Ballynagapogue Rd.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
1901 census:
http://www.census.nationalarchives.ie/pages/1901/Down/Clonduff/Ballinagappage/1248606/
http://www.census.nationalarchives.ie/pages/1901/Down/Clonduff/Ballinagappage/1248614/
http://www.census.nationalarchives.ie/pages/1901/Down/Clonduff/Ballinagappage/1248636/
There’s quite a few Truesdales (including variant spellings) on this Co. Down site:
http://freepages.genealogy.rootsweb.ancestry.com/%7Erosdavies/SURNAMES/T/Tr.htm
Some probate abstracts from the PRONI wills site:
Administration of the Estate of Francie Truesdale late of Ballynagappoge, Co. Down, who died 9 April 1910, granted at Belfast 14 April 1920 to Thomas Truesdale, Farmer, the brother. Effects: £373 10s 0d
Truesdale Isabella of Ballynagappoge county Down spinster died 4 December 1923 Probate Belfast 25 February to John Alexander McCracken law clerk and Hugh John Loughlin farmer. Effects £458 0s. 9d.
Probate of the Will of Sarah Truesdale late of Ballynagappog County Down Farmer who died 27 February 1900 granted at Belfast to Francis M'Combe and Stewart M'Neilly Farmer.
Letters of Administration of the personal estate of Thomas Truesdale late of Ballenagappoe County Down Farmer deceased who died 24 August 1866 at same place were granted at Belfast to Sarah Truesdale of Ballenagappoe (Newry) aforesaid the Widow of said deceased.